Synthesis of organometallic pentalenide complexes

Synthesis, Structure, and Reactivity of Magnesium Pentalenides

The first magnesium pentalenide complexes have been synthesized via deprotonative metalation of 1,3,4,6-tetraphenyldihydropentalene (Ph4PnH2) with magnesium alkyls. Both the nature of the metalating agent and the reaction solvent influenced the structure of the resulting complexes, and an equilibrium between Mg[Ph4Pn] and [nBuMg]2[Ph4Pn] was found to exist and investigated by NMR, XRD, and UV-vis spectroscopic techniques. Studies on the reactivity of Mg[Ph4Pn] with water, methyl iodide, and trimethylsilylchloride revealed that the [Ph4Pn]2- unit undergoes electrophilic addition at 1,5-positions instead of 1,4-positions known for the unsubstituted pentalenide, Pn2-, highlighting the electronic influence of the four aryl substituents on the pentalenide core. The ratio of syn/anti addition was found to be dependent on the size of the incoming electrophile, with methylation yielding a 60:40 mixture, while silylation yielded exclusively the anti-isomer.

Connect Four: Tetraarylated Dihydropentalenes and Triarylated Monocyclic Pentafulvenes from Cyclopentadienes and Enones

In search of novel pentalenide ligands for use in organometallic chemistry and homogeneous catalysis, we report the scope of a straightforward base-promoted Michael annulation of cyclopentadienes with α,β-unsaturated ketones that allows the introduction of symmetrical as well as unsymmetrical aryl and alkyl substitution patterns including electron-donating as well as electron-withdrawing substituents. More than 16 examples of various isomers of 1,3,4,6-tetraarylated dihydropentalenes have been synthesized in isolated yields of up to 78%, representing a substantial expansion of the range of dihydropentalene scaffolds known to date. Double bond isomerization between the two pentacyclic rings in 1,2-dihydropentalenes with electronically different substituents occurred depending on the polarization of the molecule. The melting points of the air-stable dihydropentalenes decrease, and their solubilities in organic solvents improve with increasing substitution and decreasing symmetry of the molecule. A competitive pseudo-retro-aldol pathway produces 1,3,6-triarylated monocyclic pentafulvenes as side products in yields of 9-68%, which can be cleanly isolated (8 new examples) and used for other synthetic purposes, including separate cyclization to other dihydropentalenes.